This invention is generally directed to carrier compositions, and more specifically the present invention is directed to carrier compositions containing organoalkoxysilane coatings. Also embraced within the scope of the present invention are eleectrostatographic imaging systems utilizing developer compositions containing carrier particles coated with organoalkoxysilanes. Such developer compositions are unique since they can be utilized to develop electrostatic latent images of either a positive polarity or negative polarity, that is the toner compositions can be charged positively, or negatively, depending, for example, on the carrier coating. Further, the triboelectric properties of the carrier particles of the present invention can be controlled, and/or the conductivity of the carrier core surface can be varied as desired, depending for example, on the organoalkoxysilane employed.
The formation and development of electrostatic latent images on imaging surfaces, such as photoconductive materials is well-known. In one known method, electrostatic latent images, which are formed by placing a uniform electrostatic charge on a photoconductive insulating layer, followed by exposing the layer to a light and shadow image to dissipate the charge on the area of the layer exposed to the light, is developed by depositing on the photoconductive layer image toner particles. These particles are attracted to those areas of the photoconductive layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. Subsequently, the powder image may be transferred to a support surface such as paper, and the image may be permanently fused thereto, utilizing heat for example. Numerous different methods are known for applying the toner particles to the electrostatic latent image including cascade development, magnetic brush development, and the like. In these systems, generally, a developer composition comprised of relatively larger carrier particles, containing finely divided toner particles electrostatically clinging to their surface, is conveyed to and rolled or cascaded across the electrostatic latent image member. The composition of the toner particles is selected so as to possess a triboelectric polarity opposite to that of the carrier particles, thus, as the mixture cascades or rolls across the photoconductive member the toner particles are electrostatically deposited and secured to the charged portion of the latent image, and are not deposited on the uncharged or background portions of the image. Most of the toner particles which are accidentally deposited in the background area are removed by the rolling carrier particles, due primarily to the greater electrostatic attraction between the toner particles and carrier particles than between the toner particles and the discharge background. The carrier particles and unused toner particles can then be recycled.
Carrier particles may consist of numerous substances, either coated or uncoated, providing they have the appropriate triboelectric properties, and are of the appropriate conductivity. Thus, for most uses, the carrier materials employed, or the coatings thereon, should have a triboelectric value commensurate with the triboelectric value of the toner particles, so as to enable electrostatic adhesion of the toner particles to the carrier particles, and subsequent transfer of the toner particles from the carrier particles to the image on the photoreceptor surface. Additionally, the triboelectric properties of the carrier particles should be relatively uniform to allow the carrier particles to attract toner particles, and allow the deposition of such toner particles. Further, the materials employed as carrier particles should generally have an intermediate hardness, so as to avoid scratching of the photoconductive surface upon which the electrostatic image is initially placed, while at the same time being sufficiently hard to resist and withstand the forces to which the particles are subjected during recycling. Furthermore, the use of carrier particles that are comprised of brittle materials, which cause either flaking of the surface or particle breakup under the forces exerted on the particles during recycle, is known. Such flaking causes undesirable effects in that, for example, relatively small flake particles will eventually be transferred to the copy surface, thereby interferring with the deposited toner composition, and thus causing imperfections in the final copy image. Flaking of the carrier particles surface can also cause the resulting particles to have non-uniform triboelectric properties when the carrier particle is comprised of a core material different from the surface coating thereon, and this can result in an undesirable non-uniform pickup of toner particles by the carrier particles, and non-uniform deposition of the toner particles on the electrostatic latent image. Accordingly, the type of materials useful for carrier particles, or for coatings thereon, although possessing the appropriate triboelectric properties are limited because other physical properties which they possess may cause the undesirable results mentioned herein.
It has also been disclosed in the prior art that it would be highly desirable to alter the triboelectric properties of carrier cores to provide for the use of desirable toner compositions, while at the same time retaining the other desirable physical properties of the carrier particles. The alteration of the triboelectric properties of the carrier particles by applying a surface coating thereon has been found particularly useful since, with such coatings it is possible to alter the triboelectric properties of the carrier particles made from materials having desirable physical properties, while also utilizing materials previously not suitable as carrier particles. Thus, for example, carrier particles having desirable physical properties, with the exception of hardness can be coated with a material having the desired hardness, as well as other physical properties, rendering the resulting products useful as carrier particles.
Disclosed in U.S. Pat. No. 4,039,463, are electrostatographic developers comprising a carrier particle coated with a co-polymer of N-vinylcarbazole and trialkoxysilane and/or a triacetoxyvinyl silane. Apparently, such coatings alter the triboelectric properties of the carrier particles to accommodate them to desirable toner compositions, and further, such particles can be prepared in a simplified method. U.S. Pat. No. 4,039,331 contains a similar teaching, with the exception that the carrier particles have an outer layer thereon of a vinylpyridine, and an organosilicone carrier coating. While these improved carrier coatings have been found to be useful for certain applications, in some instances, the coatings are non-uniform in thickness, and the triboelectric properties, as well as the conductivity of the carrier particle varies over extended periods of time, thus resulting in carrier surfaces which degrade with use. Accordingly, there continues to be a need for carrier particles, as well as developer compositions containing such particles, wherein the coating thereon is substantially uniform in thickness, and where the surface conductivity of the carrier particles, as well as the triboelectric properties thereof can be controlled as desired, and wherein such properties remain constant over extended periods of time. Additionally, there continues to be a need for carrier particles wherein the surface thereof does not degrade with use.